The present invention relates to magnetic particles and a magnetic carrier for an electrophotographic developer comprising the magnetic particles, and more particularly, to magnetic particles for use as an electrophotographic magnetic carrier in an electrophotographic developer, which have an excellent durability and a stable charging property, an electrophotographic magnetic carrier for an electrophotographic developer, and an electrophotographic developer using the electrophotographic magnetic carrier.
In electrophotographic developing methods, a photosensitive member composed of a photoconductive material such as selenium, OPC (organic semiconductor), a-Si or the like has been used to form an electrostatic latent image thereon by various means. Then, by using a magnetic brush method or the like, a toner having a polarity reverse to that of the latent image is attached thereon to form the latent image by the electrostatic force.
As is well known in the art, in the above developing methods, there have been used support particles called a magnetic carrier. The magnetic carrier acts for imparting an appropriate positive or negative electrical quantity to the toner by frictional electrification, and transferring the toner into a developing zone near the surface of the photosensitive member by a developing sleeve in which magnets are accommodated, using the magnetic force thereof.
In recent years, the electrophotographic developing method has been widely applied to copying machines or printers. In these apparatuses, it has been demanded to meet various requirements including not only reproduction of thin lines, small characters, photographs, color originals or the like, but also a high image quality, a high image grade, a high copying or printing speed, a continuous image formation or the like. The requirements for these properties have been estimated to become increased more and more in future.
In order to satisfy not only the applicability to various objectives but also the high image quality and the high image grade, the reduction in a particle size of the toner particles and the magnetic carrier particles, has been studied. In particular, it has been strongly demanded to provide magnetic carrier particles having an average particle size as small as 10 to 50 xcexcm.
On the other hand, in order to satisfy the high copying or printing speed and the continuous image formation, it has been strongly demanded to enhance the durability of these particles as developer. In the case of the magnetic carrier, there has been proposed such a method which comprises iron particles obtained by a mechanical pulverization method, an electrolytic method, a reduction method, a heat-decomposition method, a sintering method or the like; granulating and then heat-sintering various ferrite fine particles or magnetite fine particles to form granulated sintered particles; dispersing magnetic particles or magnetic particle and non-magnetic particles in a binder resin to form composite particles (hereinafter referred to merely as xe2x80x9cmagnetic core particlesxe2x80x9d); and then coating the surfaces of the obtained magnetic core particles with various resins. The above magnetic carrier has been already put into practice.
There is no end of a demand for the enhancement in properties of the electrophotographic developers. In order to continuously obtain a clear image, it is desired that the charge amount of the magnetic carrier is kept unchanged and stable even after the magnetic carrier is used for a long period of time. Specifically, when the magnetic carrier is used for a long period of time, there arises such a problem that the coating resin layer is peeled off from the surfaces of the magnetic core particles, so that the charging property of the magnetic carrier is deteriorated, whereby the magnetic carrier cannot impart an appropriate charge to the toner. Therefore, it has been demanded that the coating resin layer can be prevented from being peeled off from the surfaces of the magnetic core particles in order to enhance the durability of the magnetic carrier, thereby allowing the magnetic carrier to show a more stable charging property.
Hitherto, in order to enhance the durability of the magnetic carrier, there have been proposed a magnetic carrier obtained by coating the surfaces of magnetic core particles with a silicone resin (Japanese Patent Publication (KOKOKU) No. 2-3181(1990), Japanese Patent Application Laid-Open (KOKAI) Nos. 62-66269(1987) and 3-242657(1991), etc.); a magnetic carrier obtained by coating the surfaces of magnetic core particles with a silicone resin containing a silane-based coupling agent (Japanese Patent Application Laid-Open (KOKAI) No. 5-107819(1993), etc.); or the like.
At the present time, it has been strongly required to provide an electrophotographic magnetic carrier having an excellent durability and a stable charging property. However, such a magnetic carrier has not been obtained yet.
That is, in the production of the above-mentioned conventional magnetic carriers, when the coating resin layer is formed on the surfaces of the magnetic core particles, fatty acid metal salts, especially organic tin compounds, have been used together with the silicone resin in order to cure the resin. The more the amount of the organic tin compound used is increased, the more the resin can be cured more readily. As a result, it becomes possible to form a uniform and satisfactory coating resin layer without causing the aggregation between the magnetic carrier particles. However, when the amount of the organic tin compound used is as large as not less than 0.4% by weight based on the weight of the resin solid content, the obtained coating resin layer becomes brittle, so that upon a long-term use of the magnetic carrier, the coating resin layer tends to be peeled off from the surfaces of the magnetic core particles, resulting in change in charge amount, i.e., unstable charge amount of the magnetic carrier.
On the other hand, when the amount of the organic tin compound used is reduced, it is difficult to cure the coating resin and, therefore, to form a uniform and satisfactory coating resin layer on the surfaces of the magnetic core particles. In addition, there arises such a problem that upon forming the coating resin layer or upon subsequent heat-treatments, the magnetic carrier particles are aggregated together, thereby deteriorating the yield. This phenomenon becomes more remarkable in the case where it is intended to sufficiently and uniformly coat the magnetic core particles having a small particle size especially not more than 50 xcexcm, with a silicone resin.
As a result of the present inventor""s earnest studies, it has been found that by coating each surface of the magnetic core particles with a silicone resin composition comprising at least one metal alkoxide, at least one silane-based coupling agent and a silicone resin, the obtained magnetic particles are useful as an electrophotographic magnetic carrier for an electrophotographic developer. The present invention has been attained on the basis of the finding.
It is an object of the present invention to produce magnetic particles having an excellent durability and a stable charging property with a high yield without using any organic tin compound.
It is another object of the present invention to provide an electrophotographic magnetic carrier for an electrophotographic developer which has an excellent durability and, therefore, a stable charging property.
It is a further object of the present invention to provide an electrophotographic developer having an excellent durability.
To accomplish the aims, in a first aspect of the present invention, there are provided magnetic particles having an average particle size of 10 to 200 xcexcm. which comprise magnetic core particles, and a coating layer formed on each of said magnetic core particles, comprising at least one metal alkoxide represented by the general formula (I):
(RO)nMxe2x80x83xe2x80x83(I)
wherein R is a C1 to C16 alkyl group; M is Al, Ti, Na, K, Ca, Zn or Fe; and n is an integer of 1 to 4, at least one silane-based coupling agent and a silicone resin.
In a second aspect of the present invention, there is provided a magnetic carrier for an electrophotographic developer which comprises magnetic particles having an average particle size of 10 to 200 xcexcm, which comprise magnetic core particles, and a coating layer formed on each of said magnetic core particles, comprising at least one metal alkoxide represented by the general formula (I):
(RO)nMxe2x80x83xe2x80x83(I)
wherein R is a C1 to C16 alkyl group; M is Al, Ti, Na,
K, Ca, Zn or Fe; and n is an integer of 1 to 4, at least one silane-based coupling agent and a silicone resin.
In a third aspect of the present invention, there is provided a developer comprising a toner and a magnetic carrier which comprises magnetic particles having an average particle size of 10 to 200 xcexcm, which comprise magnetic core particles, and a coating layer formed on each of said magnetic core particles, comprising at least one metal alkoxide represented by the general formula (I):
(RO)nMxe2x80x83xe2x80x83(I)
wherein R is a C1 to C16 alkyl group; M is Al, Ti, Na,
K, Ca, Zn or Fe; and n is an integer of 1 to 4, at least one silane-based coupling agent and a silicone resin.
Various conditions for carrying out the present invention are described below.
First the magnetic particles according to the present invention are described.
The magnetic particles according to the present invention have an average particle size of usually 10 to 200 xcexcm. When the average particle size is less than 10 xcexcm, there is caused such a phenomenon that a toner is firmly adhered onto the surfaces of the magnetic particles, so that the charging property inherent to the magnetic particles is lost, i.e., a so-called spent toner. On the other hand, when the average particle size is more than 200 xcexcm, it is difficult to obtain a clear image. In particular, in order to obtain images having a more high quality and a more high grade, the average particle size of the magnetic particles are preferably 10 to 100 xcexcm, more preferably 10 to 50 xcexcm.
As the magnetic core particles used in the present invention, there may be used any kind of the magnetic core particles described hereinbefore.
As the granulated sintered particles, there may be used magnetic particles such as ferrite particles containing at least one element selected from the group consisting of lithium, manganese, magnesium or the like or magnetite particles. Specific examples of the preferred fine particles may include lithium-manganese ferrite, lithium-manganese ferrite, magnesium ferrite and copper-zinc ferrite.
As the composite particles, there may be used those particles obtained by granulating a mixture composed of a resin, magnetic fine particles such as the above-mentioned ferrite fine particles or magnetite fine particles and, if required, non-magnetic fine particles such as hematite fine particles, by a kneading and pulverizing method or a polymerization method. In order to obtain a magnetic carrier having a further enhanced durability, the use of composite particles having a specific gravity as low as especially 2 to 4, is preferred. Also, in order to obtain such a magnetic carrier having a high magnetization value, the use of the granulated sintered particles is preferred.
Incidentally, the magnetic fine particles or non-magnetic fine particles used upon the production of the composite particles as the magnetic core particles, may have any particle shape including a spherical shape, a plate-like shape, an acicular shape or the like. The average particle size of the magnetic fine particles or the non-magnetic particles is preferably 0.05 to 5.0 xcexcm. Further, in order to improve the properties of these particles such as dispersibility in resins, the magnetic fine particles or non-magnetic fine particles may be surface-treated with a coupling agent or the like to impart a hydrophilic property thereto.
The magnetic core particles may also have any particle shape such as a spherical shape, a granular shape, a plate-like shape or the like.
The average particle size of the magnetic core particles is usually 8 to 195 xcexcm, preferably 10 to 100 xcexcm. When the average particle size of the magnetic core particles is less than 8 xcexcm, the particle size of the obtained magnetic particles becomes less than 10 xcexcm. On the other hand, when the average particle size of the magnetic core particles is more than 195 xcexcm the particle size of the obtained magnetic particles becomes more than 200 xcexcm.
The coating resin composition used for the magnetic particles according to the present invention, comprises a silicone resin, a metal alkoxide and a silane-based coupling agent. As to the silicone resins, in the consideration of the durability of the obtained magnetic particles, the ratio of trifunctional silicone (hereinafter referred to merely as xe2x80x9cTxe2x80x9d) to bifunctional silicone (hereinafter referred to merely as xe2x80x9cDxe2x80x9d) is preferably in the range of 95:5 to 40:60, more preferably 95:5 to 50:50.
The amount of the coating resin composition is usually 0.05 to 10% by weight based on the weight of the magnetic core particles. When the amount of the coating resin composition is less than 0.05% by weight, the obtained coating resin layer tends to become insufficient and non-uniform, so that it may be difficult to enhance the durability of the magnetic particles. On the other hand, when the amount of the coating resin composition applied is too large, the obtained coating resin layer tends to be peeled off from the surfaces of the magnetic core particles, so that it may be difficult to produce a magnetic carrier having a stable charging property. The amount of the coating resin composition is preferably 0.1 to 10% by weight, more preferably 0.2 to 5% by weight based on the weight of the magnetic core particles.
The metal alkoxide of the coating resin composition used in the present invention, is represented by the general formula:
(RO)nM
wherein R is a C1 to C16 alkyl group; M is Al, Ti, Na, K, Ca, Zn or Fe; and n is an integer of 1 to 4.
In the consideration of industrial or economical uses, the R is preferably a C2 to C8 alkyl group, more preferably a C2 to C4 alkyl group. In order to further enhance the durability of the coating resin layer, the M is preferably Al or Ti. Specific examples of the metal alkoxides usable in the present invention, may include aluminum-tri-n-butoxide (n=4, M=Al), aluminum-tri-ethoxide (n=2, M=Al), aluminum-tri-sec-butoxide (n=4, M-Al), aluminum-tri-isopropoxide (n=3, M=Al), titanium-tetra-n-butoxide (n=4, M=Ti), titanium-tetraethoxide (n=2, M=Ti), titanium-tetra-iso-propoxide (n=3, M=Ti) or the like.
The amount of the metal alkoxide used is preferably 0.05 to 0.4% by weight, more preferably 0.05 to 0.35% by weight based on the solid content of the silicone resin. When the amount of the metal alkoxide used is less than 0.05% by weight, the curing speed of the silicone resin may be low, so that the magnetic carrier particles tend to be agglomerated together, resulting in low yield. On the other hand, when the amount of the metal alkoxide used is more than 0.3% by weight, the obtained coating resin layer may become brittle, resulting in deteriorated durability thereof.
As the silane-based coupling agents used in the coating resin composition of the present invention, there may be exemplified coupling agents containing an amino group, an epoxy group, a vinyl group, a mercapto group, a halogen atom and/or an alkyl group therein. Specific examples of the silane-based coupling agents may include amino-containing silane-based coupling agents such as xcex3-aminopropyl trimethoxysilane, N-xcex2-aminoethyl-xcex3-aminopropyl trimethoxysilane, N-xcex2-aminoethyl-xcex3-aminopropylmethyl dimethoxysilane, N-phenyl-xcex3-aminopropyl trimethoxysilane or the like; epoxy-containing silane-based coupling agents such as xcex3-glycidoxypropylmethyl diethoxysilane, xcex2-3,4-epoxycyclohexyl trimethoxysilane, xcex3-glycidoxypropyl trimethoxysilane or the like; vinyl-containing silane-based coupling agents such as vinyl trichlorosilane, vinyl triethoxysilane, vinyl-tris(xcex2-methoxy) silane or the like; halogen-containing silane-based coupling agents such as dimethyl dichlorosilane, methyl trichlorosilane, allyl dimethyl chlorosilane, allyl phenyl dichlorosilane, benzyl dimethyl chlorosilane, bromomethyl dimethyl chlorosilane, xcex1-chloroethyl trichlorosilane, xcex2-chloroethyl trichlorosilane or the like; mercapto-containing silane-based coupling agents such as xcex3-mercaptopropyl trimethoxysilane; or alkyl-containing silane-based coupling agents such as trimethyl silane or the like. In the case where the charge amount of a negative toner is required to increase, the use of the amino-containing silane-based coupling agents is preferable. Also, in the case where the charge amount of the toner is to be kept unchanged, the use of the epoxy-containing silane-based coupling agents is preferable.
The amount of the silane-based coupling agent used is preferably 0.1 to 20.0% by weight, more preferably 1 to 15% by weight based on the solid content of the silicone resin. When the amount of the silane-based coupling agent used is less than 0.1% by weight, the curing speed of the silicone resin may be low, so that it may be difficult to form the aimed coating resin layer having an excellent durability, and the obtained magnetic particles tend to agglomerate together. On the other hand, when the amount of the silane-based coupling agent used is more than 20.0% by weight, the obtained coating resin layer may become brittle, resulting in deteriorated durability, so that the obtained magnetic carrier tends to show an unstable charging property.
In the coating silicone resin composition used in the present invention, at least two of the metal alkoxide, the silane-based coupling agent and the silicone resin may be interacted to each other.
Next, the process for producing the magnetic particles, is explained.
As described above, the magnetic particles according to the present invention, can be obtained by diluting the silicone resin composition composed of the silicone resin, the metal alkoxide and the silane-based coupling agent with an organic solvent such as toluene or the like so as to adjust the solid content thereof to 5 to 30% by weight; and then adding to the magnetic core particles a coating solution which is prepared by adjusting the amounts of the above respective components added such that the gelation time of the silicone resin composition becomes in the range of 2 to 5 hours, thereby coating each surface of the magnetic core particles with the coating solution. Almost a whole amount of the thus applied coating solution is deposited over the surfaces of the magnetic core particles, thereby forming a coating resin layer on the magnetic core particles.
When the solid content of the coating solution is less than 5% by weight, the removal of the solvent such as toluene, etc., may need a long period of time, resulting in industrially and economically disadvantageous process. On the other hand, when the solid content of the coating solution is more than 30% by weight, it may be difficult to form a sufficient and uniform coating resin layer composed of the silicone resin composition on the surfaces of the magnetic core particles. When the gelation time is less than 2 hours, the viscosity of the coating solution itself may be increased, so that it may be also difficult to form a sufficient and uniform coating resin layer composed of the silicone resin composition on the surfaces of the magnetic core particles. On the other hand, when the gelation time exceeds 5 hours, the magnetic core particles tend to be agglomerated together.
The amount of the coating solution added is preferably 0.05 to 10.0% by weight (calculated as solid content) based on the weight of the magnetic core particles. When the amount of the coating solution added is less than 0.05% by weight, there is a tendency that the magnetic core particles are insufficiently and non-uniformly coated with the silicone resin composition. On the other hand, when the amount of the coating solution added is more than 10.0% by weight, the obtained magnetic carrier may show a too high electrical resistance, thereby causing deteriorated images such as charge-up or the like.
The magnetic particles according to the present invention have (1) a true specific gravity of usually 2 to 7, preferably 2.5 to 4.5; (2) a volume resistivity of usually not less than 106 xcexa9xc2x7cm, preferably 107 to 1015 xcexa9xc2x7cm; (3) a saturation magnetization value of usually 10 to 90 emu/g, preferably 20 to 90 emu/g; and (4) a durability (change in charge amount) of usually not more than 15%, preferably not more than 10%.
The important point of the present invention is such a fact that the magnetic particles obtained by coating each surface of the magnetic core particles with the silicone resin composition comprising the silicone resin, the metal alkoxide and the silane-based coupling agent, can show an excellent durability and a stable charging property.
The reason why the magnetic particles according to the present invention can show an excellent durability, is considered as follow. That is, the coating resin layer and the magnetic core particles are firmly adhered to each other, and the coating resin layer is effectively prevented from being deteriorated because any organic thin compound is not used therein, so that the peeling-off of the coating resin layer can be inhibited even after being used for a long period of time.
The reason why the magnetic particles can be produced with a high yield even though the magnetic core particles used have a small particle size, is considered as follows. That is, since the magnetic core particles are sufficiently and uniformly coated with the silicone resin composition so as to eliminate an exposed surface portion thereof, the obtained magnetic particles can be prevented from being agglomerated together.
Thus, the magnetic particles according to the present invention can exhibit an excellent durability and is free from the peeling-off of the coating resin layer even after being used for a long period of time. In addition, the magnetic particles show a stable charging property and, therefore, are suitable as an electrophotographic magnetic carrier for electrophotographic developer.
Further, since the magnetic particles are prevented from being agglomerated together upon forming the coating resin layer or upon subsequent heat-treatments especially even though the magnetic core particles used have a small particle size, the magnetic particles according to the present invention can be produced with a high yield and is, therefore, industrially and economically advantageous.
The electrophotographic magnetic carrier for electrophotographic developer according to the present invention shows an excellent durability and a stable charging property.
The electrophotographic developer according to the present invention shows an excellent durability and achieves a high copying and printing speed and continuous image formation in the electrophotographic developing method.